Operons and gene regulation in bacteria Flashcards
Operons
Genes in an operon are transcribed as a group and have a single promoter.
Regulatory DNA sequences
Something each operon contains that act as binding sites for regulatory proteins that promote or inhibit transcription
Inducible operons
Operons that can be turned on by the presence of a particular small molecule
Repressible operons
Operons on by default but can be turned off by a small molecule
gene regulation
mechanisms for controlling which genes get expressed and at what levels
What is found in the operons of bacteria
Related genes are often found in a cluster on the chromosome, where they are transcribed from one promoter as a single unit
Cluster of genes under the control of a single promoter
Operons
Regulatory proteins
Attach to regulatory DNA sequences that control how much operon is transcribed
Repressors
Regulatory proteins that bind to pieces of DNA called operators. When bound to the operator it reduces transcription by blocking RNA polymerase from moving forward on the DNA
Activators
Regulatory protein increases the transcription of the operon (by helping RNA polymerase bind to the promoter) when bound to its DNA site
Structure of lac operon
Three genes lacZ, lacY, and lacA which are transcribed as a single mRNA under one promoter
lac operon also contains a number of regulatory sequences which are regions of DNA in which particular regulatory proteins can bind, controlling transcription of the operon
These 3 sites are the promoter, operator and CAP binding site
Promoter
Binding site for RNA polymerase, the enzyme that performs transcription
Operator
Negative regulatory site bound by the lac repressor protein. The operator overlaps with the promoter, and when the lac repressor is bound, RNA polymerase cannot bind to the promoter and start transcription
CAP binding site
Positive regulatory site bound by catabolite activator protein. When CAP is bound to the site, it promotes transcription by helping RNA polymerase bind to the promoter
Lac repressor
protein that represses (inhibits) transcription of the lac operon. It does this by binding to the operator, which partially overlaps with the promoter.
What happens to the lac repressor when lactose is and isn’t present
When it is unavailable: The lac repressor binds tightly to the operator, preventing transcription by RNA polymerase
When it is available: The lac repressor loses its ability to bind DNA
allolactose
Isomer (rearranged version) of lactose that causes the change in lac repressor.
When lactose is available it is converted to allolactose which binds to lac repressor and makes it change shape so it could not bind DNA
It is an example of an inducer, small molecule that triggers expression of a gene or operon
lac operon
condiered an inducible operon because it is usually turned off (repressed) but can be turned on in the presence of inducer allolactose
Catabolite activator protein (CAP)
CAP binds to region of DNA just before the lac operon and helps RNA polymerase attach to the promoter, driving high levels of transcription
cyclic AMP
Regulates CAP since it is not always active
It is a hunger signal made by E.coli when glucose levels are low. cAMP binds to the CAP, changing its shape and making it able to bind DNA and promote transcription
Without cAMP CAP cannot bind DNA and is inactive
Two conditions for lac operon to be expressed at high levels
Glucose must be unavailable: Since cAMP binds to CAp, making CAP bind to DNA. Bound CAp helps RNA polymerase attach to the lac operon promoter
Lactose must be available” If lactose is available the lac repressor will be released from the operator (by binding of allactose) Allows RNA polymerase to move forward on the DNA and transcribe the operon
trp operon
group of five genes that encode biosynthetic enzymes for the amino acid tryptophan
tryptophan
Amino acid needed by bacteria to build proteins
trp operon structure
five genes that encode enzymes for trytophan biosynthesis
Promoter (RNA polymerase binding site)
Operator (binding site for repressor protein)
trp repressor
regulatory protein that binds to the DNA of the operator. As a result it keeps the operon from being transcribed by physically getting in the way of RNA polymerase, the transcription enzyme.
How the presence of tryptophan effect transcription
trp repressor only binds and blocks transcription when tryptophan is present. This is because the tryptophan attaches to the repressor molecules and changes their shape so it becomes active
When there is little tryptophan in the cell, the trp repressor is inactive
corepressor
small molecule that switches a repressor to its active state
